It is known to fabricate micro-hotplates on a silicon substrate consisting of a micro-heater formed within a thin membrane layer (made of electrically insulating layers) that is formed by front or back-etching of part of the substrate. Such micro-hotplates can be used to provide a local temperature (from ambient temperature to 700° C.) with low power consumption (typically from a few mW to hundreds of mW) for use as infra-red sources/emitters or flow sensors, or can be coated with a catalyst or sensing material and used as gas sensors. There is a large number of such devices designed and reported using several different materials for the heater or the membrane. The heater itself can be a MOSFET (EP 0 953 152), polysilicon, or metal resistive heater.
A well known reliability issue with metal heaters is caused by electromigration, which at high temperatures results in an increase in resistance over time and can eventually lead to failure of the device. For example, several studies have been shown platinum heaters failing due to electromigration (e.g. Courbat et al. “Reliability improvement of suspended platinum-based micro-heating elements,” Sensors and Actuators A 142 (2008) 284-291).
Electromigration occurs due to the momentum transfer between electrons and ions causing the movement of atoms in the presence of high current densities and the mean time to failure (MTTF) is exponentially dependent on temperature according to Black's equation (based on an Arhenius process). This results in an undesirable increase in resistance over time.
For integrated circuits, this reliability issue due to electromigration has been addressed by increasing the width of the metal tracks, which are carefully designed using Black's equation considering the current flow and the operating temperature. As a result, the effective current density through the metal tracks has been reduced. However, heaters in micro-hotplates operate at much higher temperatures and the required width of the metal tracks is often not feasible due to size constraints. In addition, an increase in the width of the metal track can also lead to high power consumption of the micro-hotplate as these tracks have to be connected from a hot spot to a pad outside the membrane. Therefore there will be an increase in thermal conduction losses resulting in an undesirable effect. There is believed to be a need for another method to improve the reliability of such metal heaters.
It has already been demonstrated that bidirectional current causes much less electromigration than a unidirectional current as disclosed in e.g. Liew et al, “Electromigration interconnect lifetime under AC and pulse DC Stress”, Proceedings of IEEE Reliability Physics Symposion 1989, pp. 215-219; Maiz, “Characterisation of electromigration under bidirectional and pulsed unidirectional (PDC) currents”, Proceedings of IEEE Reliability Physics Symposion 1989, pp. 220-228. Typically, most tracks in a micro-electronic circuit carry a unidirectional current due to constraints of circuit design (for example due to the direction of current flow in transistors or diodes) and so cannot benefit by the use of bidirectional current. The cited studies have been done to improve the design of existing tracks that already use bidirectional, or pulse voltage (such as AC signal lines or clock lines), rather than investigating specifically the effect of bidirectional current in an integrated circuit.
The use of a bidirectional current in micro-gas sensors has been suggested in various studies. For example, there are instances, in the field of gas sensors, where a bidirectional current is applied to the sensing material, but not to the heater itself. The intention in these cases is to avoid polarization effects in the sensing material, and the heater in these cases is driven by a DC signal. For example, Garcia-Guzman, “Design and simulation of a smart rationmetric ASIC chip for VOC monitoring”, Sensors and Actuators B November 2003, 232-243, describes a circuit for switching current polarity applied to the sensing material, but the circuit drives the heater by a DC current.
US 2006/0154401 suggests the possibility of having two heaters in a micro-hotplate with one of the heaters driven by a DC voltage while the other is driven by a low frequency AC signal. However, this is specifically done to modulate the temperature of the micro-hotplate. The DC heater in the system described in US 2006/0154401 could still suffer from electromigration.
Bidirectional current has also been suggested for other devices. U.S. Pat. No. 7,330,336 describes the use of bidirectional current to reduce the effect of electromigration in the heating element of disk drives.
However none of these references teaches the use of bidirectional current to drive a heater of the micro-hotplate to reduce the effect of electromigration such that the reliability of the micro-hotplate is improved.
It is an object of the present invention to reduce the effect of electromigration for improving the reliability of the micro-hotplates.